Structural vibration is one of the key performance limiting phenomena in many types of advanced machinery, such as space launch vehicle shrouds, all types of jet and turbine engines, robots, and many types of manufacturing equipment. Because structural vibration depends on many factors that are not easily modeled, such as boundary and continuity conditions, as well as the disturbance environment, it is impossible to design a machine from the first prototype that will meet all vibration requirements. This means that the final steps in analyzing and suppressing vibration are accomplished after the actual production unit has been completed.
To address this shortfall, it is known to incorporate vibration analysis and suppression systems into equipment. In general, a typical vibration analysis and suppression system includes a multitude of vibration sensors and vibration actuators that are installed on-board the equipment in selected locations. The system also includes a control system that transmits control signals in accordance with a vibration suppression algorithm to the actuators during normal operation of the equipment to mechanically suppress the vibrations. Using a feedback loop, the sensed vibration information is fed back to the control circuitry, which adjusts the control signals in response to dynamic conditions.
It is also known to incorporate vibration analysis devices into equipment for the purpose of performing non-destructive testing (i.e., testing that does not destroy the equipment). For example, sensors can be incorporated into aircraft to measure flow and combustion induced vibrations in turbines or combustion housings of propulsion systems, can be incorporated pre-forms, concrete and other structures that require cure-monitoring, or can be incorporated into equipment to monitor damage (e.g., delamination) that may present as a change in vibration characteristics.
Significant to the present invention, piezoelectric sensors and actuators are utilized extensively to detect and/or suppress vibrations in equipment. Such piezoelectric devices can be incorporated into the host structure of the equipment as plates that can be embedded within the host structure or externally applied to the host structure as patches. When used as a sensor, a piezoelectric plate contracts and expands along a plane parallel to the surface of the plate (in the x- and y-direction) in response to vibrations induced within the piezoelectric plate via the host structure, which in turn, induces an electrical field in a plane perpendicular to the surface of the plate (in the z-direction), creating a voltage potential between the top and bottom surfaces of the piezoelectric plate. In a similar manner, when used as an actuator, a piezoelectric plate contracts and expands along a plane parallel to the surface of the plate (in the x- and y-direction) in response to a voltage potential between the top and bottom surfaces of the piezoelectric plate that induces an electrical field induced in a plane perpendicular to the surface of the plate (in the z-direction), which in turn, induces a vibration in the host structure. Whether used as a sensor or an actuator, the magnitude of the voltage potential on the top and bottom surfaces of the piezoelectric plate will be proportional to the magnitude of the contraction/expansion of the piezoelectric plate, and thus, the vibrations of the host structure. Thus, the nature of the vibrations sensed within the host structure can be determined via analysis of the voltage potential, and the nature of the vibrations induced within the host structure can be controlled via the voltage potential applied to the piezoelectric plate.
To protect the very fragile piezoelectric plate from damage, and to functionally couple the piezoelectric plate between the host structure and the external circuitry that senses vibrations from the host structure and/or induces vibrations within the host structure, it is necessary to incorporate the piezoelectric plate into a package. Such packages typically include a pair of wire leads respectively coupled to the top and bottom surfaces of the piezoelectric plate to convey the voltage potential to and/or from the piezoelectric plate, and one or more layers of an electrically insulating material that encapsulate the piezoelectric plate to not only protect it from damage that might otherwise occur when dropped or mishandled, but also to electrically insulate the piezoelectric plate and wire leads from the host structure. A connector is typically mounted to the piezoelectric package, so that a cable from the control/sensing circuit can be operably coupled to the piezoelectric plate.
Piezoelectric packages are generally supplied to a user in a one size. Thus, if a single piezoelectric package is insufficient for providing the desired actuation or sensing functions at a particular location of the host structure, it may be desirable to locate multiple piezoelectric packages at this location. In this case, however, multiple cables must be connected between the control/sensing circuit and the respective connectors of the piezoelectric packages, even though the multiple packages function as a single actuator/sensor. In other words, the actuation/sensing function at a particular location of a host structure may not be easily scalable.
Oftentimes, a pair of upper and lower piezoelectric plates are incorporated into the piezoelectric package, which allows the package, when used as either an actuator or a sensor, to be operated in a specific morphological configuration, and in particular, in either a unimorph (or extensional) configuration or in a bimorph (or bending) configuration.
In a unimorph configuration, the piezoelectric plates both expand or both contract when signals of the same polarity are transmitted to the respective piezoelectric plates (assuming the package is operated as an actuator), and signals of the same polarity are received from the respective piezoelectric plates when the piezoelectric plates both expand or both contract (assuming the package is operated as a sensor). The piezoelectric package can be configured as a unimorph by coupling leads of the same polarity to the same polarized sides of the respective piezoelectric plates (e.g., positive leads to the positively polarized sides of the piezoelectric plates, and negative leads to the negatively polarized sides of the piezoelectric plates).
In contrast, in a bimorph configuration, one piezoelectric plate expands while the other piezoelectric plate contracts when signals of the same polarity are transmitted to the respective piezoelectric plates (assuming the package is operated as an actuator), and signals of the same polarity are received from the respective piezoelectric plates when one piezoelectric plate expands while the other piezoelectric plate contracts (assuming the package is operated as a sensor). The piezoelectric package can be configured as a bimorph by coupling leads of the opposite polarity to the same polarized sides of the respective piezoelectric plates (e.g., one positive lead and one negative lead respectively to the positively polarized and negatively polarized sides of one piezoelectric plate, and the other positive lead and the other negative lead respectively to the negatively polarized and positively polarized sides of the other piezoelectric plate).
When mounting the piezoelectric package to or within host structure, it is desirable that the stresses exerted by the upper and lower piezoelectric plates combine in a manner that maximizes the strain applied to the host structure when the package is operated as an actuator, or combine in a manner that maximizes the magnitude of combination of the signals received from the upper and lower piezoelectric plates when the package is operated as a sensor. This result can be achieved by judiciously selecting the morphological configuration of the piezoelectric package.
In particular, the relationship between the piezoelectric package and the host structure to which it is installed will often depend on the location of the package relative to the neutral axis of the structure to which the package is mounted. That is, any structure undergoing bending has a neutral axis plane-a plane on which no bending stress is experienced. The location of the neutral axis depends on the boundary conditions, material, and geometry of the structure, among other factors. On one side of this plane, the structure expands and on the other side, it contracts. If the piezoelectric package is located entirely on one side of the neutral axis, a unimorph configuration is better, as both piezoelectric plates will simultaneously expand or simultaneously contract in accordance with the side of the neutral axis on which it resides and the bending direction of the neutral axis. If the neutral axis extends through the piezoelectric package, however, a bimorph configuration is likely better (though it actually depends on the exact location within the piezoelectric package), as one piezoelectric plate will expand while the other piezoelectric plate will contract in accordance the bending direction of the neutral axis.
It can be appreciated that dynamic selection of a unimorph or bimorph configuration allows the user to select the most sensitive configuration in the case where the piezoelectric package is used as a sensor, or the most vibratory configuration in the case where the piezoelectric package is used as an actuator. To enable the dynamic selection of the morphological configuration, the leads, which are respectively disposed along vertical planes to connect to the top and bottom sides of both piezoelectric plates, can be laterally extended out from the piezoelectric package to form two sets of terminals (which may, e.g., be configured in a stair step fashion). Two connectors can then be respectively coupled to the terminal sets, so that a cable can be connected to the appropriate connector to dynamically place the piezoelectric package in the desired morphological configuration.
Because the relative orientation of the leads are vertically fixed, however, it is difficult to orient the respective terminal sets differently in order to enable selectivity between the unimorph and bimorph configurations using identical connectors. While it is theoretically possible to reconfigure the output/input signals at the interface of the control/sensing circuit, in practice, this would require that the device in which the control/sensing circuit is contained be modified to simultaneously input and/or output two signals, and would further require such device to be modified to allow dynamic selection between a unimorph configuration and a bimorph configuration. However, most existing devices designed to operate with piezoelectric packages are only capable of inputting or outputting a single signal. Thus, in this case, different connector configurations must be used in order to enable dynamic selectivity between unimorph and bimorph configuration. As a result, two different cables must be used with the piezoelectric package—one that uniquely couples to the unimorph connector and one that uniquely couples to the bimorph connector. Thus, the proper cable corresponding to the desired morphological configuration must be selected, which may become quite tedious, especially when multiple piezoelectric packages are to be mounted to the host structure.
Thus, there remains a need to provide a scalable and easily manufacturable piezoelectric package whose morphological configuration can be dynamically changed in a more convenient manner.